Advances in femtosecond micromachining and inscription of micro and nano photonic devices Graham Neale Smith Doctor of Philosophy Photonics Research Group Aston University October 2011 © Graham Neale Smith, 2011 Graham Neale Smith asserts his moral right to be identified as the author of this thesis. This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from the thesis and no information derived from it may be published without proper acknowledgement 1 Advances in femtosecond micromachining and inscription of micro and nano photonic devices Thesis summary This thesis has focused on three key areas of interest for femtosecond micromachining and inscription. The first area is micromachining where the work has focused on the ability to process highly repeatable, high precision machining with often extremely complex geometrical structures with little or no damage. High aspect ratio features have been demonstrated in transparent materials, metals and ceramics. Etch depth control was demonstrated especially in the work on phase mask fabrication. Practical chemical sensing and microfluidic devices were also fabricated to demonstrate the capability of the techniques developed during this work. The second area is femtosecond inscription. Here, the work has utilised the non-linear absorption mechanisms associated with femtosecond pulse- material interactions to create highly localised refractive index changes in transparent materials to create complex 3D structures. The techniques employed were then utilised in the fabrication of Phase masks and Optical 2 Advances in femtosecond micromachining and inscription of micro and nano photonic devices Coherence Tomography (OCT) phantom calibration artefacts both of which show the potential to fill voids in the development of the fields. This especially the case for the OCT phantoms where there exists no previous artefacts of known shape, allowing for the initial specification of parameters associated with the quality of OCT machines that are being taken up across the world in industry and research. Finally the third area of focus was the combination of all of the techniques developed through work in planar samples to create a range of artefacts in optical fibres. The development of techniques and methods for compensating for the geometrical complexities associated with working with the cylindrical samples with varying refractive indices allowed for fundamental inscription parameters to be examined, structures for use as power monitors and polarisers with the optical fibres and finally the combination of femtosecond inscription and ablation techniques to create a magnetic field sensor with an optical fibre coated in Terfenol-D with directional capability. Through the development of understanding, practical techniques and equipment the work presented here demonstrates several novel pieces of research in the field of femtosecond micromachining and inscription that has provided a broad range of related fields with practical devices that were previously unavailable or that would take great cost and time to facilitate. 3 Advances in femtosecond micromachining and inscription of micro and nano photonic devices Dedication To all those who have challenged, listened, read between the lines, advised and been there with me and for me with the patience of a saint. Physics is imagination in a strait jacket ~ John Moffat 4 Advances in femtosecond micromachining and inscription of micro and nano photonic devices Acknowledgements I would very much like to thank my supervisors Dr Kate Sugden (Aston University), Dr Kyriacos Kalli (Cyprus Institute of Technology) and Dr Alan Ferguson (Oxford Lasers) whose help and encouragement were invaluable in many varied ways throughout this study. I would also like to acknowledge the financial support of the ESPRC, Oxford Lasers and DTI funded projects without their support this work would not have been possible. My thanks also go to all the technical support that I received from Amplitude systemes, Aerotech and the engineers at Oxford Lasers. Who, at various times, have helped to make or keep the lasers and stages working. Thanks goes to all of my collaborators; Fiberlogix with special mention to Dr Thomas Butler, Tanya Hutter of Cambridge University, Dr Peter Wooliams and Dr Peter Tomlins during their time at NPL, Charalambos Koutsides of the Cyprus Institute of Technology, Professor Geoff Tansley, 5 Advances in femtosecond micromachining and inscription of micro and nano photonic devices Dr Laura Leslie and Graham Lee at Aston University, Ronald Neal of Plymouth University, Dr Dimitris Karnakis and Dr Martyn Knowles of Oxford Lasers. I would like to also acknowledge the collective support and patience of all my former colleagues from Aston University Photonics Research Group with a special mention to Bert Biggs, Ian Johnson, Tom Allsop, Andrew Main, Richard Reeves, Andrew Abbott, Karen Carroll, Yuen Chu, Mykhaylo Dubov, Jovana Petrovic, Yicheng Lai, Jim Harrison, David Webb and Vladimir Mezentsev who have contributed greatly to my learning, sanity and ability to query. Huge thanks must go to my family for all of their patience, support and kindness throughout my many years of education. Finally, I want to thank Dr Julia Badger for her love, patience, support and passion for research which has inspired, calmed and focused at all the key moments in the last years. 6 Advances in femtosecond micromachining and inscription of micro and nano photonic devices List of Contents Advances in femtosecond micromachining and inscription of micro and nano photonic devices .......................................................................................... 1 Thesis summary ..................................................................................................... 2 Dedication ............................................................................................................... 4 Acknowledgements ............................................................................................... 5 List of Contents ...................................................................................................... 7 List of Tables and Figures ................................................................................... 14 Figures .................................................................................................................... 14 Tables ...................................................................................................................... 29 Definition of terms ................................................................................................. 30 1 Introduction ................................................................................................. 33 1.1 Fundamental considerations of femtosecond pulse-material interaction ......................................................................................................... 35 1.2 Nonlinear excitation mechanisms ..................................................... 37 7 Advances in femtosecond micromachining and inscription of micro and nano photonic devices 1.3 Energy transfer ..................................................................................... 41 1.3.1 Modification regimes ...................................................................... 42 1.3.2 Index change ................................................................................... 42 1.3.3 Void creation .................................................................................. 45 1.4 Ablation mechanisms .......................................................................... 45 1.5 Chemical structures of fused silica and borosilicate and glass modification with femtosecond exposure ................................................... 46 1.5.1 Fused Silica ..................................................................................... 46 1.5.2 Borosilicate glass ............................................................................ 47 1.6 Experimental factors that affect the interaction .............................. 48 1.6.1 Repetition rate ................................................................................ 48 1.6.2 Write direction ................................................................................ 48 1.6.3 Polarisation ..................................................................................... 49 1.6.4 Self-focusing and filamentation ...................................................... 50 1.6.5 Pulse duration ................................................................................ 51 1.7 Materials ............................................................................................... 51 1.7.1 Non-transparent materials ............................................................. 54 1.7.2 Transparent materials .................................................................... 55 1.7.3 Heat Affected Zone ......................................................................... 57 1.8 Fibre Bragg grating and Long Period Grating theory .................... 58 1.8.1 Fibre Bragg grating theory ............................................................. 58 8 Advances in femtosecond micromachining and inscription of micro and nano photonic devices 1.8.2 Long Period grating theory ............................................................ 60 1.9 Goals of the research ..........................................................................